Method for setting an operating point of a hybrid drive of a vehicle

ABSTRACT

A method for setting an operating point of a hybrid drive of a vehicle is provided, the hybrid drive including an internal combustion engine and at least two electric motors/generators as propulsion engines, and the output shafts of the propulsion engines being operatively linkable to a drive train of the vehicle. Depending on a desired output torque and an instantaneous vehicle velocity, operating points of the electric motors/generators are set in such a way that the sum of the mechanical output and the electrical losses of all electric motors/generators of the hybrid drive equals zero.

FIELD OF THE INVENTION

The present invention relates to a method for setting an operating pointof a hybrid drive of a vehicle, the hybrid drive including an internalcombustion engine and at least two electric motors/generators aspropulsion, and the output shafts of the propulsion engines beingoperatively linkable to a power train of the vehicle.

BACKGROUND INFORMATION

In the known hybrid drives addressed here, an internal combustion engineis combined with at least two electric motors/generators, so that aplurality of drive sources for the vehicle are available. According torequirements specified by a vehicle driver, the drive sources mayoptionally feed their driving torque into a power train of the vehicle.This results in various drive configuration possibilities depending onthe driving situations, which drive configurations are used to improveride comfort and to reduce energy use, as well as to reduce pollutantemission.

In hybrid drives for vehicles, serial arrangements, parallelarrangements and mixed arrangements of internal combustion engine andelectric motors/generators are known. Depending on the arrangement, theelectric motors/generators may be connected to the power train of theinternal combustion engine directly or indirectly. For the mechanicallinkage of the internal combustion engine and/or the electricmotors/generators, it is known to arrange them in such a way that theyare mechanically linkable with each other using a transmission, e.g., aplanetary transmission, or the like, and clutches.

Optimum implementation of a driver's desired driving power from thehybrid drive requires coordinated activation of the propulsion enginesof the hybrid drive, which is accomplished by a device known as anengine control unit. In every driving situation of the vehicle, thedriver's wish must be satisfied in an optimum way with the resourcesprovided by the vehicle. Known operating strategies for hybrid drivesdefine an optimized operating point for the internal combustion engine,depending on specific input values such as driving power demand, vehiclevelocity, roadway gradient, and the like. An effort is made to operatethe internal combustion engine outside of the least efficient partialload range, insofar as possible, and to shut it off at minimum desiredpower output levels, if appropriate. In these cases, the at least oneelectric motor/generator takes over the propulsion of the vehicle. It isalso known to control the internal combustion engine along an optimumfuel consumption characteristic curve. A disadvantage of these knownoperating strategies is that the efficiencies of the electric propulsionengines and the effects of the operating behavior of the electricpropulsion engines on electric energy storage devices (batteries) areignored.

SUMMARY

The method according to the present invention has the advantage over therelated art that in the case of a hybrid drive having an internalcombustion engine and at least two electric motors/generators, theefficiencies of the electric drive components and the effects of theoperating behavior of the electric propulsion engines on electric energystorage devices are also considered in setting an operating point of thehybrid drive. Because operating points of the electric motors/generatorsare set as a function of a desired output torque and an instantaneousvehicle velocity, so that the sum of the mechanical power outputs andthe electrical losses of all electric motors/generators of the hybriddrive is zero, this produces the advantageous result that, when thehybrid drive is at rest, the electric energy storage devices remainuninvolved and their battery output is regulated to be zero.

Hybrid drives having electric motors/generators use high-performancebatteries, which are significant in cost. Because the operating strategyof the electric motors/generators regulates the battery output to bezero when the vehicle is stopped, the demand on the high-performancebatteries is reduced, and hence their total service life is extended. Inparticular, the service life of the high-performance battery may therebybe matched to the service life of the vehicle that has the hybrid drive.This results in economic savings, which significantly increase theeffectiveness of the hybrid drives. At the moment when the sum of themechanical outputs and the electrical losses of all the electricmotors/generators is zero, electric motors/generators operating asmotors are supplied with energy by at least one electric motor operatingas a generator, which in addition covers all the electrical losses ofthe electric motors/generators. This makes it possible to regulate thebattery output to be zero when the vehicle is stopped.

An example embodiment of the present invention provides that, whensetting the operating points of the electric motors/generators,attention is paid to at least one optimization criterion, e.g., minimuminstantaneous fuel consumption of the internal combustion engine. Thisallows the operating point of the hybrid drive to be chosen in such away that, in addition to low demand on the high-performance batteriesover their total service life, it is also possible to achieve the lowestpossible fuel consumption and thus the lowest possible emission ofpollutants from the hybrid drive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a hybrid drive.

FIG. 2 shows a block diagram of a method for setting an operating pointof the hybrid drive.

FIG. 3 shows an equivalent diagram of the hybrid drive.

FIG. 4 shows a block diagram of an operating strategy for the hybriddrive.

FIG. 5 shows characteristic maps for optimized operating points of theinternal combustion engine of the hybrid drive.

FIG. 6 shows an optimized characteristic diagram of the gear steps ofthe hybrid drive.

DETAILED DESCRIPTION

FIG. 1 shows a schematic view of a hybrid drive 10 of a motor vehicle.Hybrid drive 10 includes an internal combustion engine 12, and a firstelectric motor/generator 14 and a second electric motor/generator 16. Acrankshaft 18 of engine 12 and drive shafts 20 and 22 of electricmotors/generators 14 and 16 are mechanically linked to a transmissionsystem 24. Drive shaft 20 is connected to a first planetary transmission26, and drive shaft 22 to a second planetary transmission 28. A ringgear of planetary transmission 26 is connected to a speed-changetransmission 30, and a ring gear of planetary transmission 28 isconnected to a speed-change transmission 32. Speed-change transmissions30 and 32 in turn are connected to an output shaft 34 of transmissionsystem 24. Output shaft 34 is mechanically linked to a drive shaft ofthe motor vehicle, not shown.

The construction and manner of operation of such a hybrid drive 10 aregenerally known, so that the present description will not go into it infurther detail. By selectively activating engine 12 and/or electricmotors/generators 14 and 16, it is possible to deliver a differentdriving torque to output shaft 34. Thus it is possible to set differentoperating modes of hybrid drive 10. By operating a gear selector,speed-change transmissions 30 and 32 permit various gears, designatedhere as gears 1, 2, 3, 4, 5 and 6, and a reverse gear R, are engageable.Electric motors/generators 14 and 16 may each be operated in generatormode or motor mode, and are used, for example, to provide an on-boardsupply voltage for the motor vehicle and to charge a rechargeablebattery. Electric motors/generators 14 and 16 have braking devices 36and 38 associated with them, by which rotors of electricmotors/generators 14 and 16 may be mechanically braked.

FIG. 2 shows a block diagram of a portion of an engine control devicefor actuating hybrid drive 10. The engine control device includes acoordinator 40 for specifying a characteristic-diagram-based operatingstrategy for hybrid drive 10. The operating strategy sets an optimumoperating point of hybrid drive 10, as explained below.

From a transducer 42, coordinator 40 receives a signal 44 thatcorresponds to the instantaneous velocity v of the vehicle. From atransducer 46, coordinator 40 receives a signal 48 that corresponds toan output torque desired by a vehicle's driver. Transducer 46 mayoperate, for example, in coordination with an accelerator pedal, a brakepedal or an automatic driving control system of the vehicle.

From input signals 44 and 48, coordinator 40 determines signals 50, 52,54, 56 for activating engine 12, electric motors/generators 14 and 16and transmission system 24. Signal 50 contains a speed specification anda torque specification for engine 12, signal 52 contains a speedspecification and a torque specification for electric motor 14, signal54 contains a speed specification and a torque specification forelectric motor 16, and signal 56 contains a gear step specification fortransmission system 24.

To carry out the characteristic-map-based operating strategy,coordinator 40 uses characteristic maps. The starting point fordetermining these characteristic maps is the equivalent diagram forhybrid drive 10 illustrated in FIG. 3. Hybrid drive 10 includes engine12, electric motors/generators 14 and 16, and transmission system 24.Associated with electric motors/generators 14 and 16 is ahigh-performance battery 58, which is fed by electric motors/generators14 and 16 in generator mode, and which battery feeds electricmotors/generators 14 and 16 when they are in motor mode. In the normalcase, one electric motor operates in motor mode and one electric motoroperates in generator mode.

A tank 60 is provided to supply engine 12 with fuel, an instantaneousfuel consumption rate 62 being determined. Hybrid drive 10 delivers anoutput power P to output shaft 34. Output power P is a function of thevehicle velocity v (signal 44) and the desired output torque M (signal48).

On the basis of this equivalent diagram shown in FIG. 3, an optimizationcriterion is defined, which is represented, for example, by minimuminstantaneous fuel consumption rate 62.

A driving state of the vehicle is defined through the output power P,and hence through instantaneous velocity v and desired output torque M.These driving states are implemented through operating points of thedrive train, i.e., through operating points of engine 12, electricmotors/generators 14 and 16, and transmission system 24.

For the present invention, the sum of the mechanical power outputs ofelectric motors/generators 14 and 16, and the electrical losses ofelectric motors/generators 14 and 16, equals zero. This means that oneof the electric motors/generators 14 and 16 operates in generator modeand the other of the electric motors/generators 14 and 16 operates inmotor mode. In so doing, the electric motor operating in generator modesupplies the electric motor operating in motor mode with energy, and inaddition covers all the electric losses of the two electricmotors/generators 14 and 16. The result is that for this assumedsteady-state operating state the power output of battery 58 is regulatedto be zero.

From the set of all possible drive train operating points with which oneof the operating states, defined through output power P, isimplementable, coordinator 40 thus first determines all operating pointsof engine 12, electric motors/generators 14 and 16, and transmissionsystem 24 that satisfy the requirement that the sum of the mechanicaloutput and electrical losses of the electric motors/generators equalszero.

These optimized operating points of engine 12, electricmotors/generators 14 and 16, and transmission system 24 that satisfythese boundary conditions are subjected to an additional optimizationcriterion, namely, according to the example, a minimum possibleinstantaneous fuel consumption 62. This producesfuel-consumption-optimized operating points of engine 12, electricmotors/generators 14 and 16, and transmission system 24. Thesefuel-consumption-optimized operating points are stored in controlcharacteristic maps, which are used by coordinator 40. Since thesecontrol characteristic maps are derived from operating characteristicmaps of the involved units, i.e., engine 12, electric motors 14 and 16,and transmission system 24, these control characteristic maps alsoimplicitly make allowance for the operating limits of those units, suchas maximum speed or full load characteristic curves, so that they do nothave to be requested separately.

FIG. 4 shows a block diagram of the method according to the presentinvention for setting the operating point of hybrid drive 10 bycoordinator 40. First input signals 44 (instantaneous velocity v) and 48(desired output torque) are linked to a characteristic map 64 thatspecifies an optimum gear step for transmission system 24. This signal66 corresponding to the optimum gear step is fed to a gear step enablingunit 68, which enables the optimum gear step as the setpoint gear stepand issues control signal 56. The enabling of the setpoint gear step maybe made as a function of additional parameters, for example to preventshifting while in a curve, double shifting, etc. Signal 56 is providedto transmission system 24 for setting the gear step. Signal 56 is alsolinked to a characteristic map 70 to determine a setpoint operatingpoint of engine 12. The resulting signal 50 is supplied to engine 12 andto a module 72. Module 72 gates signal 50 with signal 56 and suppliessignals 52 and 54 for activating electric motors/generators 14 and 16,which correspond to their optimum operating points.

The representation in FIG. 4 makes it clear that the method according tothe present invention is easily implemented. Simultaneous calculationsof the possible operating points of the units of hybrid drive 10 fordifferent gear steps are not necessary, so that no major computingeffort is needed. The gear steps are enabled immediately after theoptimum gear step is determined, so that in the event that enabling ofthe optimum gear step is prevented, the subsequent steps to determinesignals 50, 52 and 54 do not have to be performed unnecessarily. Thecapacity that is not claimed by this may be used to search for analternate approach, for example a different gear step. Thischaracteristic-map-based operating strategy, into which characteristicmaps 64 and 70 are incorporated, produces a very reliable controlsystem, in which the resources used for monitoring the units of hybriddrive 10 may be reduced to a minimum, since the characteristic maps ofthe operating strategy already ensure that no non-permissible operatingpoints of the units are activated.

An adaptation to different hybrid drives 10, e.g., hybrid drives 10having a different number of gear levels, is easily implemented due tothe modular structure of the control system, since it is merelynecessary to adapt control characteristic map 64 and gear step enablingunit 68.

FIG. 5 shows exemplary characteristic maps 70, by which the optimizedoperating points of engine 12 are determinable. Each driving state thatis characterized by vehicle velocity v (signal 44) and the desiredoutput torque (signal 48) has a setpoint torque M_(setpoint) and asetpoint speed n_(setpoint) associated with it. These valuescorresponding to the optimized operating points are fed to engine 12 andto module 72 (FIG. 4) as signal 50 (FIG. 4).

FIG. 6 shows control characteristic map 64 for selecting the optimumgear step. It is possible here, depending on vehicle velocity v (signal44) and the desired torque M (signal 48), to implement driving statesthat are settable using different gear steps. By linking withinstantaneous fuel consumption optimization 62, one obtains theoptimized gear step characteristic map depicted for example in FIG. 6,on the basis of which signal 66 (FIG. 4) is output as a function ofvelocity v and the desired output element n.

If the above general explanations are applied to hybrid drive 10 shownin FIG. 1, a total of seven system variables result, namely torques andspeeds for the two electric motors/generators 14 and 16 and for engine12, as well as the gear step of transmission system 24 as a combinationof two gear steps of speed-change transmissions 30 and 32. Transmissionsystem 24, as the coupling element for engine 12, electricmotors/generators 14 and 16, and output shaft 34, delivers four boundaryconditions i.e., two kinematic boundary conditions for the speeds of theunits and two dynamic coupling conditions for the torques of the units.With attention to minimizing the instantaneous fuel consumption, theseboundary conditions may be taken into account in determining the optimumgear step for minimum instantaneous fuel consumption by engine 12 andsetting the output of battery 58 to zero.

1. A method for setting an operating point of a hybrid drive of avehicle, the hybrid drive including an internal combustion engine and atleast two electric motors as propulsion engines, and the output shaftsof the propulsion engines being selectively, operatively linked to adrive train of the vehicle, comprising: setting operating points of theelectric motors depending on a desired drive torque and an instantaneousvehicle velocity; wherein the operating points are set such that the sumof the mechanical output and the electrical losses of all the electricmotors of the hybrid drive is zero, and wherein one electric motoroperating in generator mode supplies the other electric motor operatingin motor mode with energy and compensates for all electrical losses ofboth electric motors, and wherein at least one optimization criterion istaken into account when setting the operating points of the electricmotors, and wherein minimum instantaneous fuel consumption rate of theinternal combustion engine is used as an optimization criterion.
 2. Themethod as recited in claim 1, wherein the operating point of the hybriddrive is set with the aid of a control characteristics map.
 3. Themethod as recited in claim 1, wherein the speed and torque of theinternal combustion engine and the electric motors, and a gear step fora transmission system of the vehicle, are specified.
 4. The method asrecited in claim 3, wherein an optimum gear step is determined, and thedetermined optimum gear step is enabled as a setpoint gear step,depending on defined operating states of the vehicle.
 5. The method asrecited in claim 1, wherein the operating point of the hybrid drive isset with the aid of a control characteristics map.
 6. The method asrecited in claim 1, wherein all possible drive train operating pointsare first determined, and from the determined possible drive trainoperating points, the operating points that take into account the atleast one optimization criterion are determined.
 7. The method asrecited in claim 6, wherein the determined operating points that takeinto account the at least one optimization criterion are stored in atleast one characteristic map, and wherein the characteristic map isaccessed for setting the operating point of the hybrid drive.
 8. Amethod for setting an operating point of a hybrid drive of a vehicle,the hybrid drive including an internal combustion engine and at leasttwo electric motors as propulsion engines, and the output shafts of thepropulsion engines being selectively, operatively linked to a drivetrain of the vehicle, comprising: setting operating points of theelectric motors depending on a desired drive torque and an instantaneousvehicle velocity; wherein the operating points are set such that the sumof the mechanical output and the electrical losses of all the electricmotors of the hybrid drive is zero, and wherein one electric motoroperating in generator mode supplies the other electric motor operatingin motor mode with energy and compensates for all electrical losses ofboth electric motors, and wherein all possible drive train operatingpoints are first determined, and from the determined possible drivetrain operating points, the operating points that take into account atleast one optimization criterion are determined.
 9. The method asrecited in claim 8, wherein the determined operating points that takeinto account the at least one optimization criterion are stored in atleast one characteristic map, and wherein the characteristic map isaccessed for setting the operating point of the hybrid drive.
 10. Amethod for setting an operating point of a hybrid drive of a vehicle,the hybrid drive including an internal combustion engine and at leasttwo electric motors as propulsion engines, and the output shafts of thepropulsion engines being selectively, operatively linked to a drivetrain of the vehicle, comprising: setting operating points of theelectric motors depending on a desired drive torque and an instantaneousvehicle velocity; wherein the operating points are set such that the sumof the mechanical output and the electrical losses of all the electricmotors of the hybrid drive is zero, and wherein one electric motoroperating in generator mode supplies the other electric motor operatingin motor mode with energy and compensates for all electrical losses ofboth electric motors, and wherein an optimum gear step is determined,and the determined optimum gear step is enabled as a setpoint gear step,depending on defined operating states of the vehicle.